On Thu, Sep 24, 2020 at 05:37:42PM +0200, Thomas Gleixner wrote: > Subject: softirq; Prevent starvation of higher softirq vectors > From: Thomas Gleixner <[email protected]> > Date: Thu, 24 Sep 2020 10:40:24 +0200 > > From: Thomas Gleixner <[email protected]> > > The early termination of the softirq processing loop can lead to starvation > of the higher numbered soft interrupt vectors because each run starts at > the lowest bit. If the loop terminates then the already processed bits can > be raised again before the next loop starts. If these lower bits run into > the termination again, then a re-raise might starve the higher bits forever. > > To prevent this, store the leftovers of the previous run in the upper 16 > bit of the local softirq_pending storage and ensure that these are > processed before any newly raised bits are handled. > > Signed-off-by: Thomas Gleixner <[email protected]> > --- > kernel/softirq.c | 58 > +++++++++++++++++++++++++++++++++++++++++++++++-------- > 1 file changed, 50 insertions(+), 8 deletions(-) > > --- a/kernel/softirq.c > +++ b/kernel/softirq.c > @@ -259,11 +259,23 @@ static inline bool __softirq_needs_break > return need_resched() || __softirq_timeout(tbreak); > } > > +/* > + * local_softirq_pending() is split into two 16 bit words. The low word > + * contains the bits set by raise_softirq(), the high word contains pending > + * bits which have not been processed in an early terminated run. This is > + * required to prevent starvation of the higher numbered softirqs. > + */ > +#define SIRQ_PREV_SHIFT 16
Note that in the case of x86, irq_start.__softirq_pending is a u16. The origin is there: 9aee5f8a7e30330d0a8f4c626dc924ca5590aba5 "x86/irq: Demote irq_cpustat_t::__softirq_pending to u16"
